Carrier system with phase discrimination



anathema I 2,l85,687

UNITED- -STATES' PATENT OFFICE I canam a sizfifikr gg mass. tfifa'ii'2tiiiii3$tafi1 its: I

porated, New York, N. Y., a corporation of v New York a Application February 11, ms, Serial No. 189,941 ii mums. (01. 110-15) The present. invention relates to carrier wave system and Figs. 3, 4 and 6 are schematic circuit transmission and has for its object a'simpliflcadiagrams showing typical ways in which the inion of the terminal apparatus required for sevention may be practiced, and Figs. 2 and show lective transmission and reception, ofa pluralcurves to be referred to in the explanatory por- 5 ity of messages over the same circuit. tions of the specifications. 5

, In the types of multiplex carrier systems in In a system of the character described valves commercial use at present, separation. of the are used at the transmitting station to separate messages in the different channels is effected by p itive from negative half waves of the carrier use of frequency-selective circuits such as band and, similarly, valves are used at the receiver to m f l r resonant circuits. One filter per chanadmit to respective receiving branches the half l he] is required at each terminal. The present Waves of respectively opposite polarity. Each of invention relates to message wave separation onthe valves at the transmitter has associated with a diilerent basis and specifically on the basis of it a message inp circuit 80 that the amplitude the phase relations distinguishing the message of the half waves which pass through each of the waves in respective channels. The message valves is varied in accordance with the respecwaves in different channels need not in such a tive message to be sent. At the receiver the 0888 o cupy mutuallyexclusive ranges but can valves admit the respective message-modulated have the same frequencies. Differences inphase waves of different po y t0 the respective relations used to distinguish the different chanceiving branches, and the receivers in the renelsmay be secured by producing a phase shift spective branches respond to the message com- 29 in the carrier as between one channel and anp other or by producing a phase shift in the mes- Such a system is diagrammatically shown in sage waves in order to distinguish channels. ex- F 1 W r at Station I the Source Of carrier amples of both of which will be given in the dewaves 10 is provided, together with two branches scription to follow. for the A and B subscriber sets or other message 25 It has been proposed heretofore to transmit" circuits. Set A is included in circuit with oneone message on carrier half waves of one polarity way devices or. valves II and I3 in a branch and another message on the opposite half waves across the line. Set B is similarly connected in of the same carrier wave. In such case the sepa-v a line branch including one-way devices or valves 30 ration of the messages at the receiver, even as- I! and I4, poled oppositely to devices II and I3. 30 suming a perfect line, has not been complete un- Similar branches are indicated by 'primed nu.- less a very broad frequency band is used. Lirnmerals at station 2. Communication can take iting the frequency band increases the cross-talk place in either direction over the line between A from one channel into the other. In such a sysand A and between B and B.

tern, therefore,.th ,t rminal apparatus, although It seems obvious from considering the system of 35 simpler than the usual type employing frequency Fig. 1 that complete separation of the messages selective filters, was simplified at the expense of at a receiving point is possible if none of the A requiring avery wide frequency range in order message is on the line during the half cycles to make possible the separation of the messages allotted to the B message and vice versa. -What 40 at the receiver. this means is that the currents used by the A 40 In accordance with a feature of this invention, message must be zero during the B message half the apparatus at opposite ends of the line for reperiods and vice versa. The Fourier analysis spectively impressing messages onthe line and shows that in order for this to happen it is necderiving the messages therefrom, have cooperatessary to transmit practically an infinite freing characteristics such as to enable effective quency band since it is necessary to have pres- 45 separation of messages at the receiver without cut a suilicient number of sine waves of harthe necessity of sending frequencies higher-than. monic frequencies to add-up to the required wave those comprised in the upper side-band oi the shape comprising only half waves of the carrier message-modulated carrier. modulated by message currents. The number of I The further features and .the various objects harmonics of the carrier wave together with 50 v of the invention wili be .made clear in the folside-bands. of the harmonics, required to give lowing detailed description of illustrative emcertaindegrees'of separation of messages in the iments aST'ShOWII by way of example in the receiver expressed indecibels has been calculated accompanying drawings. and is shown by the plot in Fig. 2. It is found I In the drawings Fig. 1 represents a prior art that in order to provide decibel reduction in 66 cross-talk it would be necessary to transmit a band of frequencies about 200 times the carrier frequency. This contrasts strongly with systems using frequency discrimination as a basis of separation, in which the total band width need be no greater than the product of the speech band by the number of channels.

Referring again to Fig. 1, unless the practically infinite frequency range indicated by Fig. 2 is to be required, it must be permissible to' have currents representing the A message flow during the half periods allotted to the B message and vice versa. In other words, it was the requirement that the current half waves of one message must fall to zero throughout the half periods of the other message that led to the condition of practically infinite frequency band. Narrowing the-band width necessitates permitting current representinglaoth messages to be on the line together.

Applicant has discovered that this can be done and that the messages can still be separated at'a receiving point if the voice channel as well as the carrier side-band channels are used in certain'ways presently to be described.

Stillreferring to Fig. 1, let it be supposed that 1 the amplitude of'the carrier wave is very large 4 compared to the message waves so that the'carresulting from the message cent. Let it also v a cut-off frequency or has .a low-pass filter in component of voice rier is effective in opening and closing the valves at the respective stations while the message waves vary the carrier amplitude only, say.',a':;few.per be supposed that. the line has it so that the frequency band transmitted is limited to the top frequency of the upper elde -band modulation of the carrier wave. Under these conditions the A message currents cannot be assumedto" be zerothroughout the B message half period's When the carrier opens the B receiving branch. therefore, some of the message currentsintended for the A subscriber are admitted and results in producing cross-talk. Applicant hasffound that this cross-talk can be just canceled by an equal wave transmitted not through the carrier channel but in the voice channel in proper relation to the side-band way message sets. A

is connected in circuit-2lj waves. Thus, the speech currents sent through the voice channel augment the speech waves sent through the carrier channels in the receivingcircuit branches for which the messages are intended and neutralize the messages sent through the carrier channels in the opposite receiving branch.

This is due to the phase relations of the speech components received by carrier in the respective branches and the speech components directly received. In this way the voice channel ofthe line is used to assist in the separation of the carrier messages. Different ways ofac this action will now be described.

Referring sto Fig, 3, line '20 is sh ing two two-way two-channel station andstation 2. Station I has voice frequency, terminal circuits 2! and 22'leading respectively-'- to subscribers stations Aland B1 or other. twodouble balanced copper-. oxide modulator 23 and a similar apparatus'llisconnected in circuit 22. These modulators maybe of the type Patent 2,086,601, July 13, 1937.- They suppress the input-wave components and transmitonly the-modulated wave components or demodulated wave components. The carrier generator through a band-pass filter 26 which-excludes the shown in my. prior line currents comprise:

position.

subscriber B1 is prevented by the 'iact t 25 is connected tothe line side-band and voice components'from the generator circuit. Since the modulator-demodulator 23 does not pass the voice, a by-pass 21 is pro- -videdf for impressing some of the original voice from set- A1 to the line, the amplitude beingcontrolled by variable pad 28. A simflar by-pass.

around.modulator-demodulator 24 is provided at 29, with a cross-over for reversing the phase indicated at 3!. Low-pass filter 82 is included in the line for limiting the frequency band transmitted and phase shifter 33 is used because the carrier source is only at one station and the phase shift at station I and the beating carrier oscillations of the line must be such as to produce proper I phase relation between received side-band waves from source 25. The phase shifter 33 builds out the line to the proper phase shift value to insure eflicient message reception at station I. This function of the phase-shifter is more fully disclosed in U. S. patent to R. W. Chesnut 1,983,537, issued December 11, 1934.

The apparatus atstation 2 is similar to that at station I except that no is needed. Corresponding elements to those of station I are indicated by the similar numbers .primed.

The operation of the system of Fig. 3 will now' be described. It will be assumed at first-that both side-bands of the modulated wave are sent modulation of the carrier by the voice from both sub-sets A1 and B1. All of these waves are impressed'on the circuits at station 2.

. Considering first the waves that reach sub-set A1, these-comprise the detected side-band components from 23' and the directly received voice components that traverse the by-pass circuit 21.. It is assumed that as to the waves from A1 the demodulated side-band-components and 'dicarrier source is pro-- vided and no additional line filter or phase shifter.

modulators 23'and 24, respecrectly transmitted voice components are in aiding phase relation whereas those from sub-set B1 are, due to the cross-over at 3!, in-pha'se op- By proper. control of .-pads 28, 30 and 28",.the strength of these various ma fcontrolled to reduce the cross-talk the re quirf amount. The reception of waves by sub-'- set 31' is in the same manner, the direct voice from A1v undergoing a phase reversal at ll while'that from 31 undergoes two reversals, at 3! and 3|. j Direct transmission between subscriberj'Ai and t any transmissionthrough the two 23 and 24 is neutralized by the transmission through the voice circuits 2! and 29. By virtue of the cross-over at 3| the currents transmitted through the two paths cancel eachother.

If only the lower side-bands of the modulated has acut-ofl' In this case the 1 waves are utilied, filter. 82 above the carrier frequency.

pads 28, 30 and 28', 30', must be readjusted to transmit only about half as much voice'as "78 components band waves coil 43 and a pair of linear formerly since the strength of the detected sideis'only halt what it was inthe case double side-band transmission. Fig. 4 illustrates another type of system, according to the invention, capable of supplying the right amount of direct voice component at a receiving point to neutralize the cross-talk from the opposite talker. In this figure the line 40 terminates at station I in two line or channel branches H and 42 leading to respective; subscribers stations or other message circuits A: and B2.- Channel I rectifiers 44 such as diodes or copper-oxide rectifiers having the characteristic of ,infinite impedance for impressed, negative voltages and finite linear impedance for impresse'clpositive voltages. Chann'el branch" has a' similar. repeating coil 45 and row" band-pass Filter passes modula ted' wave but suppresses harmonics of the linear rectifiers l6 oppositely poled in relation to rectifiers M. The source of carrier waves is at 41 and is connected'to line 40 through narfilter 48. Phase shifter 49 and low-pass fllter 50 serve, in general, the same purposes as described in connection with Fig. 3. both side-bands of the carrier carrier.

At station 2 the 'line 40 leads to circuit branches including repeating coils 5| and 52 capable of transmittingvoice and carrier frequencies.

These repeating coils are associ 'te'd with modulators53 and '51 comprising suitably biased diodes or copper-oxide rectifiers or similar devices having the characteristic of for negative polarity of impressed wave and finite impedance for positive polarity of impressed wave, such that the current varies as some powerof the voltage, for I -Modulators 53 and 54 are oppositely example as the square of the voltage. poled. and biased by the batteries shown. On the drop side of these modulators are voice circuit branches 55 and 56 leading to subscribers stations or other message terminals A2 and B2.

The operation of the system of Fig. 4 andthe manner in which the unwanted voice component is canceled out in the receiver will be more clearly understood by the aid of some mathematical fore mulae.

It is known that when voice is impressed on a linear rectifier together with a large amplitude carrier wave which in effect opens and closes the valve, the transmitted wave is the product of the voice wave byea functionwhich has the value +1 during the positive the value zero during the carrier, this function being:

wave in circuit branch M to be Aces a I in branch 42 to be B ,cos b, the two carrier-modulated waves transmittedirom. station I are and a v When-these-two waves 'are received at station 2 they are again multiplied by the Function 1,

branch 4i includes repeating "tude related to that band components in infinite impedance half cycle of the carrier and the negative half cycles of as a; f om 2 and 3 by Com 0 are respectively 4 and 5 as fo1lows:.

- For branch 56 these same'two expressions are multiplied by es 6] 2 1r and give the audible components 1 1 1 1 BC[-,- cos b-lrlaC[ cos a (7) The direct voice component transmitted. over the line from sub-set B2 has its maximum ampliof the sum of the two sidethe proportion of 1 t Z I 2 we 1r I If these were impressed on the receiving branch leading to the A2 receiver and if this branch had linear, rectifiers instead of square law devices,

they would only partially cancel each other for they would emerge in the proportion of I it.

leaving a considerable amount of cross-talk. But

when these two components of the B2 channel voice are impressed on the square law devices 53,

as above stated, giving, for

their values are changed, according to Expresbe in the proportion of sio'n 5, to

which when multipliedby the Function 1 emerge in the proportion of I nee 5;

This effect is illustrated graphically in Fig. 5

where the portion of the sine wave C indicates one cycle only of the carrier wave the positive half of which is extended in dotted lines to indicate the instant of maximum modulation by speech iromi B2. half 'Of the mitted .to circuit .Af'b'y rectifiers 53.

carrier wave is the half that is ad- The .A2

It is assumed that the positive,

signal modulationis not'shown, Subtraction of the carrier lobe-from the total dotted extension gives, tor greaterv convenience of comparison, the

half wave BA obmaximum amplitude.

. in arbitrary constants. This may be taken as the energy content of that iragment of the speech from B2 which is comprised in both side-bands.

ofthe modulated wave from linear rectifier 45' but confined to one-half period or the carrier for cycle of the carrier Ci.

waves-614s provided only at illustrative purposes. This component is represented in Expression .3 by the term {-3 cos b. cos

Expression 3 shows that there is a direct voice component (represented by the term B cos b), which is illustrated as V2. in Fig. 5, this being the fragment of the direct' speech that is admitted to receiving circuit A2 V2 is shown with opposite sign to BA. circuit 53 were a linear rectifier similar to rectifier 46, the impulses BA and V2 would simply be multiplied by the Function 1 and would not cancel each other in the However, circuit 53 multiplies these as well as by Function 1. converts the impulsesBA and V2, respectively, to BA and V2 (Fig. 5) with maximum amplitudes of l I r 7 respectively, which when multiplied by Function line 40 from modulators band components and in the ratio of 53 and 54 include side direct-speech components shown at V2 and BA in Fig. 5 at their arrival at station I. The received compol. 2 cos 01 causing the wanted components cross-talk components to cancel Expressions 6 and 7.

It is .seen, therefore,

cross-talk suppression is and to add and the as indicated in that a high degreefoi' obtained without havand 80. A source of carrier station I.

branches through source is connected to the line narrow,band-pass filters 68 and 69. Double balspectively through low-pass opposite ends of hybrid coil during the positive half V rect voice currents ammo? anced modulators .63 and 64 are connected refilters 65 and 68 to terminals for line 60 and a line balance impedance at 13. One-way devices 10 and H are directly connected across the line branches. The

fiers as in the previous figures. Similar elements may be used in the modulators 63 and. 64.

Since modulators 63 anced, message waves are not directly transmitted through them but only the side-band comhigher frequencies. Considering the side-bands coming from filter 65, these are rectified in linear rectifier 10 which preferably has infinite imped ance for waves of one polarity and linearfinite line 6O side-band and voice frequency compo nents from each transmitter, the voice components resulting from the rectifications at 70 and II. I

0n the assumption of large carrier amplitude so that the modulation is always less than one hundred per. cent, the transmission from this point on and reception at that 01' Fig. 4.

and 64 are double ba1- 12 which has line In Fig. 8 all modulators can be linear rectifiers in view of the fact that three of these devices are traversed in tandem, as will appear.

Thus, in Fig. 6-the side-band components, di-

arrive at station 2. Thevwanted side-bands and speech cooperate as in Fig. 4 to reinforce each other in one channel and cancel each other in the other. Sub-set I! 6| substantially free of cross-talk and sub-set U0 similarly hears talker interference. i

- Zlflll iii place in the following manner. from sub-set As to the line is to station I takes The message sent or 66. The message wave arriving at H is multi plied by -Z cos c-% cos 30+ producing the signal wave 1 1 I A. cos a'[; co: 6-; cos c -l-vo1ce receives the message from at 62 without cross-talk l and large amplitude carrier and higher harmonic side-bands. Inasmuch as u and means in each receiving circuit to receive I the side-band components cancel, no voice is produced by the double balanced demodulator 6| into sub-set Bo. Multiplication of the assumed arriving signal wave at rectifier III by -;-+3;(cos cos 3 0+ causes the side-band components to add and produce the desired v voice signal in sub-set As. The circuits that have been shown for illustration are to be consideredas typical rather than as limiting, the scope of the invention being defined by the claims.

What is claimed is: I

1. The method of multiplex carrier wave trans-- mission comprising sending a plurality of message modulated carrier waves and simultaneously sending the message current waves to a distance, there impressing the sent waves on each of a plurality of receiving circuits, and producing in the various waves such phase relations that each message modulated wave and the message current wave representing the same message augment each other in one only of said receiving circuits while neutralizing each others effects in other than said one receiving circuit.

2. In the selective reception of messages, the method comprising utilizing a voice frequency component to augment the detected component of a voice modulated carrier wave-in one receiver circuit and causing said voice frequency component to neutralize the detected component of said voice modulated carrier wave in a receiving circuit intended for a different message.

3. The method of selective transmission of messages comprising sending each message simul-' taneously through a common carrier channel and through a common message channel, utilizing the phase relations between the message currents representing the same message received from the two channels to reinforce each other in producing message currents in one receiver and utilizing the phase relations between the message currents representing another message received from the two channels to neutralize each other in said receiver.

4. In a multiplex tron system, means to modulate the same carrier wave separately by a plurality of message currents and to transmit the modulated currents to a distance,,means to transmit simultaneously the corresponding message currents, a plurality of receiving circuits,

a message component of the modulated wave and the corresponding message current in aiding relation and the corresponding components represe'nting another message in neutralizing relation.

5. In a multiplex on system, a source of message wave components, means to modulate a carrier wave by the message wave components, to produce side-band components, means to send both the side-band components and the message .wave components over the system, one receiver cumulatively responsive to the two components 1y responsive to other components so sent representing another message, and a second receiver cumulatively responsive to the two components so sent representing said other message and differentially responsive to the' two components representing said one message, the components diflerentially received neutralizing each other in each case.

6. In. a signaling system, means at a sending station to separately modulate a carrier wave by two speech messages simultaneously, means to transmit. side-band components representing both such modulations, and means to send along with the side-band components the speech message wave components representing the two speech messages, means for separately receiving the two speech messages comprising two receiving circuits, each producing additive effects from the components of one message transmitted as side-band components and as speech components and each causing the speech components and sideband components representing the other message to cancel each other.

'Z. In a multiplex signaling system, a source of carrier waves, a pair of paths containing one-way devices-at one station for controlling the opposite half waves of the carrier in accordance with two difierent messages to be sent, a pair of paths at the other station containing one-way devices for separating the currents representing the respective messages, and receivers associated with the respective last-mentioned paths, the devices atone station being linear rectifiers and those at the other station having substantially pure product modulator characteristics throughout their transmitting regions whereby cross-talk in said system is reduced.

8. In a multiplex signaling system, a source of carrier waves, means at one station to modulate half waves of .the carrier by one message. means thereat to modulate the opposite half waves of the carrier by another message, and at a difierent station, means to detect the said messages received irom the first station, the said means at one of said two stations being square law rectiflers and the said means at the other of said stations being linear rectiilers whereby crosstalk in said system is reduced.

9. The method of selectivetr'ansmission and reception of messages comprising sending each message simultaneously through a common car- ROBERT S. OARUTHIBS. .i 

